This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This study will be open for subjects who successfully complete the PKU-004 study (GCRC #1086) or the PKU-006 to assess the long-term safety of taking Phenoptin. The study subjects will be known responders to the study medication, Phenoptin. This study will allow study subjects to take study medication for up to three years.